Retinal diseases often result in blindness due to loss of post-mitotic neuronal cells. Among the retinal diseases are rod or cone dystrophies, retinal degeneration, retinitis pigmentosa, diabetic retinopathy, macular degeneration, Leber congenital amaurosis and Stargardt disease. In most retinal degenerations, cell loss is primarily in the outer nuclear layer which includes rod and cone photoreceptors. With the loss of post-mitotic neuronal cell populations, an exogenous source of new cells as a replacement for photoreceptor cells is needed.
A potential replacement source of photoreceptor cells includes stem cells. Early studies incorporated the use of mouse cells, mouse stem cells or heterogeneous populations of retinal progenitor cells as a possible source of cells for replacement of lost photoreceptors. These early studies described transplantation of photoreceptor precursor cells from postnatal day 1 mouse retina (Maclaren et al. Nature 444(9):203-207, 2006), in vitro generation of retinal precursor cells from mouse embryonic stem cells (Ikeda et al. Proc. Natl. Acad. Sci. 102(32):11331-11336, 2005), generation of retinal progenitor cells from postnatal day 1 mouse retinas (Klassen et al. Invest. Ophthal. Vis. Sci. 45(11):4167-4175, 2004), implantation of bone marrow mesenchymal stem cells in an RCS rat model of retinal degeneration (Inoue et al. Exp. Eye Res. 8(2):234-241, 2007), production of retinal progenitor cells, including ganglion cells, amacrine cells, photoreceptors wherein 0.01% of the total cells expressed S-opsin or rhodopsin, bipolar cells and horizontal cells, from the H1 human embryonic stem cell line (Lamba et al. Proc. Natl. Acad. Sci. 10(34):12769-12774, 2006) and induction of induced pluripotent stem cells (iPS) from human fibroblasts to produce retinal progenitor cells (Lamba et al. PLoS ONE 5(1):e8763. doi:10.1371/journal.pone.0008763). None of these approaches produced a homogeneous population of photoreceptor progenitor cells or photoreceptor cells for implantation. None of these approaches produced a homogeneous population of photoreceptor progenitor cells or photoreceptor cells that showed in vivo rod or cone function (e.g., detectable by conferring improvements in visual acuity). Supplies of donor-derived tissue from which photoreceptors and photoreceptor progenitors may be isolated (such as cadavers, fetal tissue, and live animals) are limited. Stem cells can be propagated and expanded in vitro indefinitely, providing a potentially inexhaustible source of non-donor derived cells for human therapy. Differentiation of stem cells into a homogeneous population of photoreceptor progenitors or photoreceptors may provide an abundant supply of non-donor derived cells for implantation and treatment of retinal diseases.